math/z3: Improve port

• Use upstream release archive
• Change USES= compiler:c++11-lang to compiler:c++20-lang
• Do some slight rearragement of port Makefile to more closely follow Porters Handbook
• Respect CPUTYPE set by framework

Note:
Upstream is inconsistent about versioning but 4.13.4.0 is defined
in project files.

PR: 283858
Reviewed by: arrowd (maintainer)